Electric discharge machining wire with optimized surface coating

ABSTRACT

The invention concerns an electrode wire comprising a metal core ( 16 ) coated with a zinc surface cladding ( 17 ) whereof the thickness (E) ranges between 2 and 4 μm, thereby providing a better compromise between machining speed, precision of workpieces and surface condition of the machined workpieces.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to electrode wires for sparkerosion machining.

[0002] Spark erosion is used to machine an electrically conductive part,by generating sparks between the part to be machined and an electricallyconductive wire. The electrically conductive wire is moved in thelengthwise direction of the wire in the vicinity of the part, and ismoved progressively in the transverse direction relative to the parteither by moving the wire in translation or by moving the part intranslation.

[0003] The sparks progressively erode the part and the wire. Thelongitudinal movement of the wire maintains at all times a wire diametersufficient to prevent its breaking in the sparking area. The relativemovement of the wire and the part in the transverse direction enablesthe part to be cut or its surface to be treated, if necessary.

[0004] Wire spark erosion machines include means for holding andtensioning a length of wire in the vicinity of the part to be machinedin a sparking area filled with a dielectric such as water, means formoving the wire longitudinally in the sparking area, means forgenerating a sparking current between the wire and the part to bemachined, and means for producing relative movement of the wire and thepart transversely to the longitudinal direction of the wire.

[0005] Spark erosion machining is generally effected in a succession ofsteps. During a first step, high energy sparks are generated in themachining area, for fast cutting of the part to confer on itsubstantially its final dimensions. During a second step, moderateenergy sparks are generated in the sparking area, and a second pass iscarried out to correct the geometry of the part and thus to eliminategeometry defects caused by the high energy sparks during the first step.An additional step at moderate energy may be necessary to correct thegeometry further. During a third finishing step, low energy sparks aregenerated in the sparking area and a finishing pass is carried outduring which the means for producing relative transverse movement of thewire and the part track the already cut shape of the part, and the lowenergy sparks thus correct the roughness of the part.

[0006] Many types of spark erosion wire are currently available, and canbe divided into two main families.

[0007] In the first family, the wires have a generally homogeneoustransverse structure, consisting of copper, brass, tungsten ormolybdenum, for example. The alloy chosen must satisfy electricalconductivity and mechanical strength requirements. Conductivity isrequired to feed energy into the sparking area. Mechanical strength isrequired to prevent the wire from breaking in the sparking area. Ifpossible, the alloy is chosen so that the wire exhibits a behaviorfavorable to erosion, that is to say so that the wire enables erosion tobe carried out at high speed. The maximum erosion speed of a wire is thespeed limit beyond which the wire breaks if the sparking energy isincreased to accelerate erosion.

[0008] Another important parameter is the conformity of the machinedpart in terms of its accuracy and surface state.

[0009] As a general rule, each wire structure confers a machining speed,a machining accuracy and a surface state.

[0010] It is difficult to find an alloy that optimizes the machiningspeed, machining accuracy and surface state parameters simultaneously.Brass wires have been thus proposed containing 35% to 37% of zinc, andthis constitutes an economically acceptable compromise, but is achievedat the cost of a relatively low erosion speed.

[0011] The second family of spark erosion wires comprises coated wires,that is to say wires consisting of a metal core covered with a usuallyhomogeneous metal or alloy surface layer. During wire spark erosionmachining, the electrical arc thus struck between the surface of thewire and the surface of the part, through the dielectric such as water,does not reach the center of the wire, which would otherwise break. Itis the coating of the wire that is worn away.

[0012] The benefit of coated wires is that the core of the wire can bechosen for its electrical and mechanical properties, and the coating canbe chosen for its erosive and contact resistance properties.

[0013] The document FR 2 418 699 thus proposes coating the core with analloy of zinc, cadmium, tin, lead, bismuth or antimony. It teaches thatthe coating improves the machining speed. The examples teach a coatingapproximately 15 μm thick.

[0014] However, it is found that the above kind of spark erosion wirecannot achieve satisfactory machining accuracy and a satisfactorysurface state at the same time.

[0015] The document EP 0 185 492 describes an electrode wire whose coreis covered with a layer of a hot-diffused alloy of copper and zinc 0.1μm to 15 μm thick containing from 10% to 50% of zinc. Fabricationinvolves a complex process including many process steps. The documentdoes not teach how to obtain a good surface state on the part to bemachined by the electrode wire. The machining accuracy aimed for andachieved is of the order of 10 μm, which is inadequate for present-dayrequirements.

[0016] The document EP 0 521 569 teaches to manufacture an electrodewire by electrolytic deposition onto a metal core, and wire drawing.Codeposition of two or more metals to a thickness from 10 μm to 20 μmprevents scaling of the surface layer. Fabrication involves a complexprocess including many process steps, including a codeposition step inwhich the relative proportions of the metals must be strictlycontrolled. And the document does not teach how to obtain a good surfacestate on the part to be machined by the electrode wire.

[0017] The document JP 62 213924 A describes an electrode wire having acore coated with 0.5 μm to 4 μm of zinc or zinc alloy. A smoothinterface is necessarily provided between the core and the coating toobtain a good surface state of the part to be machined. Producing thesmooth interface increases the cost of manufacturing the electrode wire.

[0018] The present invention is the result of research into optimizingthe structure of a spark erosion wire to combine fast erosion, veryaccurate machining, and reduced electrode wire manufacturing cost.

[0019] To reduce the manufacturing cost, the invention starts from theidea that a wire with a surface layer of non-diffused zinc is simpler toproduce on an industrial scale. There is no need to monitor and to adaptthe concentration of metals in the zinc bath, and the metal lends itselfwell to wire drawing and final sizing operations.

[0020] It was first observed, confirming the teaching of the document FR2 418 699, that increasing the thickness of a zinc coating acceleratesspark erosion.

[0021] A second, and surprising, observation was that, on some sparkerosion machines, obtaining a fine surface state compromises thegeometrical accuracy of the parts, for the same wire, but that thegeometrical accuracy of the parts can be significantly improved by ajudicious choice of the thickness of the zinc coating.

[0022] It was also found that using an electrode wire with a continuousnonoxidized zinc coating significantly improves the surface state of themachined part.

SUMMARY OF THE INVENTION

[0023] Thus, the problem addressed by the present invention is first todesign a new electrode wire structure for spark erosion machining thatoptimizes the machining speed and machining accuracy parameters, andreduces the cost of manufacturing the electrode wire.

[0024] Another object of the invention is to propose an economicalmethod of manufacturing the above kind of electrode wire, and amachining method that optimizes the parameters previously cited.

[0025] The invention preferably also aims to improve the surface stateof the machined part.

[0026] To achieve the above and other objects, the invention provides anelectrode wire for spark erosion machining, comprising a metal corecoated with a surface layer of zinc, wherein the thickness of thesurface layer of zinc is from 2 μm to 4 μm. It is particularly easy andinexpensive to produce the zinc layer, because there is no need tocontrol its composition, which is not the case with prior art wiresmanufactured by codeposition of a plurality of metals and/or thermaldiffusion.

[0027] In the context of the invention, the expression “surface zinclayer” means a layer of pure zinc in the compact hexagonal or terminalsolid solution phase, or an alloy containing at least 90% by weight ofzinc.

[0028] The thickness of the surface layer can advantageously be chosenand varied as a function of the total diameter of the wire. Accordingly,for an outside wire diameter from approximately 0.25 mm to approximately0.30 mm, the thickness of the surface layer can advantageously be from 3μm to 4 μm. On the other hand, for an outside wire diameter of less than0.25 mm, the thickness of the surface layer can advantageously be fromapproximately 2 μm to approximately 3 μm.

[0029] Choosing this range of thicknesses has the surprising advantageof solving the problem addressed by the invention.

[0030] The surface layer is preferably continuous, in other words doesnot expose the core of the wire. This improves the reproducibility ofthe sparks in the sparking area, stabilizes the process, and makes themachined surface more regular.

[0031] A surface layer substantially free of oxidized surface areas andconsequently having a bright metallic appearance guarantees goodelectrical contact between the electrode wire and the part, whichinfluences the accuracy of the automatic detection of the position ofthe part to be machined relative to the axis of the wire during thefinishing step. This also guarantees good electrical contact between theelectrode wire and the sparking current generator. That improves thesurface state of the part.

[0032] According to the invention, an electrode wire as defined abovecan advantageously be used for a method of machining a part by sparkerosion. This method includes a step of finishing the part in which theelectrode wire having a surface layer having a thickness from 2 μm to 4μm is moved progressively to track the contour of the part and tocorrect its roughness.

[0033] The manufacture of an electrode wire in accordance with theinvention can comprise the following steps:

[0034] a. producing a temporary coating of appropriate thickness byelectrolytically depositing zinc onto a metal core blank,

[0035] b. by successive drawing and annealing operations, reducing thediameter of the wire to its final value, the appropriate thickness ofthe temporary coating and the appropriate diameter of the core blankbeing chosen such that, after drawing, the wire has the required totaldiameter and a final surface layer thickness from approximately 2 μm toapproximately 4 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Other objectives, features and advantages of the presentinvention will emerge from the following description of particularembodiments, which is given with reference to the accompanying drawings,in which:

[0037]FIG. 1 is a diagrammatic front view of a spark erosion machine;

[0038]FIG. 2 is a plan view showing the spark erosion method used by themachine from FIG. 1;

[0039]FIG. 3 is a plan view of a machined part from FIGS. 1 and 2; and

[0040]FIG. 4 is a diagrammatic perspective view to a larger scale of oneembodiment of an electrode wire according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0041] Consider first of all FIGS. 1 to 3, which show a spark erosionmachining method using a wire. The spark erosion machine shown in FIG. 1essentially comprises a machining enclosure 1 containing a dielectricsuch as water, means such as pulleys 2 and 3 and wire guides 20 and 30for holding an electrode wire 4 and tensioning it in a sparking area 5inside the enclosure 1, a part support 6 and means 7 for moving the partsupport 6 relative to the electrode wire 4 in the sparking area 5. Thepart 8 to be machined, held by the part support 6, is placed in thesparking area 5. The wire guides 20, 30 are on respective opposite sidesof the part 8 to be machined, and accurately guide the electrode wire 4.For this reason they are positioned close to the part 8 to be machined,and their diameter is slightly greater than that of the electrode wire4; for example, they have a diameter of 254 μm for an electrode wire 4having a diameter of 250 μm. The electrode wire 4 moves longitudinallypast the part 8 to be machined in the sparking area 5, as indicated bythe arrow 9. An electrical generator 10, electrically connected to theelectrode wire 4 by a line 18 and by the electrically conductive pulley2, and to the part 8 to be machined by a line 19, generates in thesparking area 5 appropriate electrical energy causing electrical arcs tobe struck between the part 8 to be machined and the electrode wire 4.

[0042] The machine includes control means for adapting as a function ofthe machining step the electrical energy, the speed at which theelectrode wire 4 moves, and the movement of the part 8 to be machined.

[0043] As can be seen in FIG. 2, by moving the part to be machined in atransverse direction shown by the arrow 11, the erosive sparks cause theelectrode wire 4 to penetrate progressively into the mass of the part 8to be machined, which is electrically conductive, and produce a slot 12.Then, by moving the part 8 to be machined in the direction of the arrow13, a perpendicular cut is produced, to obtain a final part as shown inFIG. 3, with a first machined facet 14 and a second machined facet 15.

[0044] Clearly, if the electrical generator 10 generates a highelectrical energy, sparking can be faster and thus the part to bemachined can be moved faster relative to the electrode wire 4, achievingfast machining. However, because the erosive sparks are generateddiscontinuously, increasing the speed of movement affects thegeometrical accuracy of the machined part, and affects the surface stateof the machined facets 14 and 15.

[0045] In accordance with the invention, a better compromise can beobtained by using a particular electrode wire, which is shown to alarger scale in FIG. 4. The electrode wire according to the inventioncomprises a metal core 16 whose composition is chosen as a function ofthe required mechanical and electrical properties of the electrode wire4. For example, an iron core can be used to increase the tensilestrength of the electrode wire 4, or a nickel or nickel alloy core, acopper core, or a brass core. How to make the choice will be evident tothe person skilled in the art. The metal core 16 is coated with asurface layer 17 of zinc whose thickness E is from 2 μm to 4 μm. Theaccuracy of the parts 8 to be machined is reduced if the thickness ofthe surface layer is greater than 4 μm. For example, for a steel part 50mm high, a surface layer thickness of 7 μm can lead to a dimensionalerror in the part from approximately 4 μm to approximately 6 μm, whereasa surface layer thickness less than or equal to 4 μm will lead to adimensional error of only 1 μm to 2 μm. Below 4 μm, the accuracy of theparts is virtually not affected by the thickness of the surface layer.

[0046] Then, the machining speed being an increasing function of thesurface layer thickness, there is no benefit in reducing the thicknessbelow the satisfactory range.

[0047] For this reason, in accordance with the invention, a surfacelayer thickness from 2 μm to 4 μm is advantageously chosen.

[0048] This thickness can be adapted to the outside diameter of thewire. For a wire whose outside diameter D is from approximately 0.25 mmto approximately 0.30 mm, the thickness E of the surface layer 17 can befrom 3 μm to 4 μm. For a wire whose outside diameter D is less than 0.25mm, the thickness E of the surface layer 17 can be from approximately 2μm to approximately 3 μm.

[0049] The surface layer 17 is preferably continuous, as shown in FIG.4; in other words its lateral surface does not expose the metal core 16of the wire.

[0050] The surface layer 17 is preferably substantially free of oxidizedsurface areas, and consequently has a bright metal appearance.

[0051] In this regard, it has been found that the presence of electrodewire surface irregularities, or a surface oxide layer, causes irregularsparking, especially during finishing steps when machining at low power.This results in a machined part surface state for which the roughness Rais generally greater than 0.28 μm. On the other hand, with a nonoxidizedwire and a regular zinc layer, it has been possible to obtain a machinedpart surface state whose roughness Ra is less than 0.18 μm, or even aslow as 0.14 μm if the generator of the spark erosion machine isoptimized.

[0052] A method comprising the following steps can advantageously beused to manufacture an electrode wire in accordance with the inventionof the above kind:

[0053] a. producing a temporary coating of zinc with an appropriatethickness by electrolytic deposition onto a metal core blank ofappropriate diameter; the appropriate thickness of the temporary coatingis greater than the thickness of the final surface coating to beobtained; similarly, the appropriate diameter of the metal core isgreater than the final diameter of the core in the wire to be obtained;and then

[0054] b. by means of successive drawing and annealing operations,reducing the wire to the final diameter and obtaining the requiredmechanical properties, such as ultimate tensile strength, elongation atthe ultimate tensile strength, straightness. To prevent the formation ofoxides, the annealing steps are of short duration or carried out in aneutral atmosphere.

[0055] Thus, the appropriate temporary coating thickness is chosen sothat, after drawing, the thickness E of the final surface layer 17 isfrom approximately 2 μm to approximately 4 μm. Similarly, theappropriate diameter of the core blank is chosen so that, after drawing,the diameter of the core is such that the total diameter D of the wireis the required total diameter.

[0056] The invention teaches using an electrode wire as definedhereinabove to machine a part by spark erosion. During such machining, apart finishing step is carried out during which wire having a surfacelayer 17 with a thickness E from 2 μm to 4 μm is moved progressively totrack the contour of the part and to correct its roughness. Using a wirewhose surface zinc layer 17 has a continuous thickness fromapproximately 2 μm to approximately 4 μm and is substantially free ofoxides represents the best compromise between machining speed, theaccuracy of the parts and their surface state.

[0057] The present invention is not limited to the embodimentsexplicitly described, but encompasses variants and generalizationsthereof within the scope of the following claims.

1. An electrode wire for spark erosion machining comprising a metal core(16) coated with a surface layer (17) of zinc, characterized in that thethickness (E) of the surface layer (17) of zinc is from approximately 2μm to approximately 4 μm.
 2. An electrode wire according to claim 1,characterized in that, for an outside wire diameter (D) fromapproximately 0.25 mm to approximately 0.30 mm, the thickness (E) of thesurface layer (17) of zinc is from 3 μm to 4 μm.
 3. An electrode wireaccording to claim 1, characterized in that, for an outside wirediameter less than 0.25 mm, the thickness (E) of the surface layer (17)of zinc is from 2 μm to 3 μm.
 4. An electrode wire according to any ofclaims 1 to 3, characterized in that the surface layer (17) of zinc iscontinuous, so that it does not expose the metal core (16) of the wire.5. An electrode wire according to any of claims 1 to 4, characterized inthat the surface layer (17) of zinc is substantially free of oxidizedsurface areas, and consequently has a bright metallic appearance.
 6. Useof an electrode wire according to any of claims 1 to 5 for a method ofmachining a part (8) by spark erosion, which use is characterized inthat it includes a step of finishing the part (8) to be machined inwhich the electrode wire (4) having a surface layer (17) of zinc havinga thickness (E) from 2 μm to 4 μm is moved progressively (11, 13) totrack the contour of the part (8) to be machined and to correct itsroughness.
 7. A method of manufacturing a wire according to any ofclaims 1 to 5, which method is characterized in that it includes thefollowing steps: a. producing a temporary coating of zinc of appropriatethickness by electrolytic deposition onto a metal core blank ofappropriate diameter; and b. by successive drawing and annealingoperations, reducing the diameter of the wire to its final value, theappropriate thickness of the temporary coating and the appropriatediameter of the core blank being chosen so that, after drawing, the wirehas the required total diameter (D) and a thickness (E) of the finalsurface layer (17) of zinc from approximately 2 μm to approximately 4μm.
 8. A wire manufacturing method according to claim 7, characterizedin that the annealing steps are of short duration or are carried out ina neutral atmosphere, to prevent the formation of oxides.